Heat exchanger with modified diffuser surface

ABSTRACT

A diffuser comprising an inlet passage, an outwardly diverging sidewall, and an outlet. The inlet, outwardly diverging sidewall and outlet each have an inside surface that defines an interior through which a gas or fluid passes in a flow direction from the inlet to the outlet. Wherein at least a region of the inside surface includes a plurality of individual surface features that are configured to reduce or eliminate unwanted boundary layer separation of gas or fluid that passes thereover.

FIELD OF INVENTION

This invention relates generally to the field of heat exchangers and, more particularly, to heat exchangers that include a diffuser that is specially engineering having a modified inside surface to help control pressure loss and gas or fluid flow distribution within the heat exchanger.

BACKGROUND OF THE INVENTION

The present invention relates to heat exchangers that are generally configured comprising a number of internal fluid or gas passages disposed within a surrounding body. In an example embodiment, the internal passages are designed to accommodate passage of a particular fluid or gas in need of cooling, and the body is configured to accommodate passage of a particular cooling fluid or gas, i.e., coolant, used to reduce the temperature of the fluid or gas in the internal passage by heat transfer through the structure of the internal passages. A specific example of such a heat exchanger is one referred to as a shell and tube exchanger, which can be used in a variety of applications including cooling the exhaust gas from internal combustion engines.

Such heat exchangers are often used in the automotive industry for the purpose of cooling exhaust gas purpose of cooling pressurized air for example from a turbocharger or supercharger. In such applications, packaging constraints are often an issue for such heat exchangers, since the available space for such a heat exchanger proximate to the engine is often at a premium. Such heat exchangers generally include an inlet and outlet diffuser at inlet and outlet ends of the heat exchanger that operate to direct the gas or fluid in need of cooling into and out the heat exchanger. A tube bundle comprising a plurality of internal passages is interposed between the inlet and outlet diffusers and accommodates the passage of the to be cooled gas or fluid therein.

For heat exchangers that are placed into service to cool a gas, the inlet and outlet diffusers represent wasted space from a heat transfer perspective, as the gas that is passed through the diffusers are not in heat transfer contact with the heat exchanger cooling medium. As a consequence of needing to meet tight spatial demands, heat exchangers that are constructed for use in such gas cooling applications incorporate the use of a short diffuser, which can also provide greater space for the heat exchanger core. However, the use of such short diffusers are known to a create boundary layer separation of the gas flow entering the heat exchanger that operates to increase the pressure drop and disrupt the flow distribution of gas through the heat exchanger.

In some applications, such as high temperature exhaust gas coolers, the uneven flow distribution resulting from the use of such short diffusers can also cause a reduced resistance to thermal fatigue and reduced heat transfer efficiency. The higher pressure drop through the heat exchanger as a result of such short diffusers can also result in less gas flow through the exchanger or higher fuel consumption to overcome the pressure drop.

Much of the pressure loss and flow distribution problems associated with heat exchangers comprising such short diffusers arise from boundary layer separation of the gas that is flowing therein. FIG. 1 is a cross-sectional side view of a conical diffuser 10 of a heat exchanger placed into a gas cooling service. As can be seen, after the gas has passed through an opening 11 of diffuser 10, large zones 12 of eddy losses occur and are present near the transitional conical smooth inside wall surfaces 13 of the diffuser downstream of the opening 11. This zone of eddy loses that occur along the wall surface create both a disruption of the gas flow and a pressure loss within the heat exchanger prior to entering the heat exchanger core or tube bundle.

Attempts have been made to control the character of gas flow in this region through the use of ribs or fins that are positioned along the diffuser surface. In such attempts, however, the use of ribs or fins only affected the boundary separation perpendicular to the gas flow direction. Additionally, the interior surfaces comprising the ribs or fins used in such attempts can be difficult and time consuming to manufacture.

It is, therefore, desired that a heat exchanger be constructed comprising a diffuser that is specially engineered in a manner that reduces or eliminates the above-mentioned boundary layer separation phenomenon for the purpose of reducing pressure loss, improving gas flow distribution through the heat exchanger, and thus improving heat exchanger performance and efficiency. It is further desired that such heat exchanger diffuser be constructed in a manner that provides space efficient packaging, and that can be manufactured in a manner that is cost effective.

SUMMARY OF THE INVENTION

A diffuser of this invention is connected to a heat exchanger for receiving a gas or fluid and for directing the received gas or fluid to the heat exchanger. The diffuser includes a body that has a gas or fluid inlet passage at one end of the body and that extends a distance therein. The body includes an outwardly diverging wall section that extends radially away from the inlet passage with axial distance from the inlet passage. The body includes an outlet that extends axially away from the outwardly diverging wall section. The body inlet passage, outwardly diverging wall, and outlet each have an inside surface that defines an interior through which a gas or fluid passes in a flow direction from the inlet passage to the outlet. At least a section of the inside surface includes a plurality of individual surface features that are disposed therealong.

In an example embodiment, the individual surface features are projections that each project outwardly a distance from the inside surface and that are positioned along at least a region of the inside surface of the outwardly diverging wall section. In a preferred embodiment, the projections are provided in the form of rounded dimples, and occupy at least about 10 percent of the surface area of the outwardly diverging wall section. The projections can be provided in rows and arranged such that the projections in one row are staggered from the projections in an adjacent row so as to avoid straight line passage of gas through the row of projections.

Configured in this manner, the surface features in the diffuser operates to prevent the formation of a large gas recirculation zone along the inside wall surface, thereby minimizing the formation of macro-boundary layer has separations and improving the gas flow and thermal transfer efficiencies of the heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood with reference to the following drawings wherein:

FIG. 1 is a cross-sectional side view of a prior art heat exchanger conical diffuser showing fluid flow vectors demonstrating boundary layer separation;

FIG. 2 is a perspective view of a heat exchanger and diffuser as constructed in accordance with the invention;

FIG. 3 is a cross-sectional side view of a section of the heat exchanger of FIG. 2 illustrating a conical diffuser embodiment;

FIGS. 4A and 4B are schematic illustrations of the gas flow within a conventional diffuser not including the surface features of this invention, and a diffuser construction in accordance with this invention comprising the surface features, respectively;

FIG. 5 is a cross-sectional side view of a section of the heat exchanger of FIG. 2 illustrating a curvilinear diffuser embodiment;

FIG. 6 is a perspective view of the diffuser of FIG. 5 illustrating its inside wall surface; and

FIG. 7 is a cross-sectional side view of a gas flow handling device constructed in accordance with the principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to heat exchangers used for reducing the temperature of an entering gas or fluid stream. The particular application for the heat exchangers of the present invention is with vehicles and, more particularly, to cool an exhaust gas stream taken from an internal combustion engine. However, it will be readily understood by those skilled in the relevant technical field that heat exchanger constructions of the present invention as described and illustrated herein are understood to be used in a variety of different applications, and thus the invention disclosed herein should not be limited to such applications

Generally, heat exchangers constructed in accordance with principles of this invention include a diffuser that has been specially engineered to have an inside wall surface configured with a plurality of surface features that are use to reduce or eliminate unwanted boundary layer separation of gas within the diffuser during heat exchanger operation. In an example embodiment, the plurality of surface features is provided in the form of dimples or outwardly projecting convex elements.

FIG. 2 illustrates a shell and tube heat exchanger 20 constructed according to principles of the invention that generally includes a tube bundle or core 22 (shown in cut away form) disposed within a surrounding shell 24, wherein the tube bundle or core comprises a plurality of individual gas or fluid passages 26 that can be provided in the form of individual tubes. The heat exchanger 20 includes a diffuser 28 that is attached to an inlet end 30 of the heat exchanger shell 24. In this particular embodiment, the heat exchanger includes an outlet end 32 that is positioned opposite from the shell inlet end 30, and that includes a flanged portion 34 that is configured to facilitate connection with a suitable gas or fluid handling device (not shown). The shell includes a coolant inlet and a coolant outlet to facilitate the passage of a suitable cooling medium through the shell.

Referring to FIGS. 2 and 3, the diffuser 28 comprises a radially outwardly projecting flange 36 at an end 38 opposite the heat exchanger shell 24 that is configured and sized to accommodate attachment with a suitable gas or fluid handling device used to transfer the gas or fluid to be cooled to the heat exchanger. The flange 36 can include means for facilitating attachment to such fluid handling device, such as openings 40 disposed axially therethough to facilitate a bolted connection therewith. Moving axially away from the flange 38, the diffuser 32 includes a neck 42 comprising a cylindrical wall surface that defines an inlet passage 44 extending axially inwardly a distance into the diffuser from the diffuser end 38. In an example embodiment, the inlet passage 44 has a constant diameter, but inlet passages configured having a non-constant diameter are also possible and understood to be within the scope of the invention. In an example embodiment, the inlet passage can be configured having a round or circular cross section. However, it is to be understood that the inlet passage 44 may be configured having other cross-sectional configurations that are characterized by a constant or variable dimension moving axially therethrough.

Moving axially away from the inlet passage 44, the diffuser 28 includes an outwardly or outwardly diverging wall section 46 that extends outwardly away with distance from the inlet passage. The outwardly diverging wall section 46 can be conical or curvilinear in configuration depending on the particular heat exchanger application. In the embodiment illustrated in FIG. 3, the diffuser outwardly diverging wall section 46 is conical. The outwardly diverging wall section 46 extends a distance to an end section 48 that is configured and sized to facilitate connection with the shell inlet end 30. The end section 48 can be configured having a constant or variable diameter extending axially therethrough, and can have a circular or non-circular cross section. In the embodiment illustrated in FIG. 3, the end section 48 has a constant diameter that is sized to match with the shell 24 to facilitate connection therewith.

It is to be understood that the dimensions of the diffuser inlet passage 44 and the outwardly diverging wall section 46 can and will vary depending on the particular heat exchanger application. For applications such as exhaust gas cooling for internal combustion engines, the length of the diffuser, and thus the length of the inlet passage and outwardly diverging wall section will be that which provides a desired balance of reduced sized to meet noted spatial packaging issues, and heat transfer efficiency.

Referring to FIG. 3, the diffuser end section 48 is configured to not only provide a desired attachment with the shell 24, but is also configured to make a desired attachment with a header 50 that is connected to the tube bundle 22. In an example embodiment, the diffuser end section 48 includes a recessed groove 52 extending a depth into the inside wall surface that is sized to accommodate a complementary end section of the header 50 therein. Constructed in this manner, the diffuser operates to fix the position of the header and tube bundle within the shell when it is attached the shell.

The diffuser inlet passage 44 and outwardly diverging wall section 46 together define an interior chamber 54 through which a gas or fluid entering the diffuser passes therethrough. It will be understood that the diffuser interior chamber 54 increases in volume as the outwardly diverging wall section 46 extends away from the inlet passage 44 and towards the end section 48 and end of the diffuser. After passing through the diffuser 28, the gas or fluid then enters and passes into the tube bundle 22 of the heat exchanger.

The diffuser outwardly diverging wall section 46 is configured having a plurality of individual surface features 56 disposed therealong. The surface features are configured and arranged along at least a region of an inside surface of the outwardly diverging wall section for the purpose of reducing the eddy losses and boundary layer separation of gas passed through the diffuser. The surface features can be positioned along the inside surface of the diffuser that includes the inlet passage 44 and the outwardly diverging wall section 46. In an example embodiment, the surface features 56 are positioned along the outwardly diverging wall section 46.

It has been discovered that the placement of the surface features along at least a region of the of the outwardly diverging wall section 46 operates to reduce and or eliminate eddy loses and boundary layer separation of gas passing through the diffuser. In an example embodiment, the surface features preferably occupy at least about 10 percent of the surface area of the outwardly diverging wall section.

The term “individual” as used herein to refer to the surface features is understood to mean that each surface feature is separate from one another and is not provided in the form of a continuous element, such as an elongate rib or a fin. The individual surface features can be provided in the form of individual projections that each extend outwardly a distance from the wall surface. A feature of using such individual projections when compared to ribs or fins is that they have are more effective at reducing the unwanted eddy currents along the diffuser inside wall surface independent of flow direction.

The projections can be configured and sized differently depending upon the particular heat exchanger application. The projections can be disposed along the inside wall surface in an ordered or random manner, i.e., having an ordered or random arrangement of repeating projections. In an example embodiment, the projections are provided in an ordered pattern that are staggered relative to one another for the purpose of avoiding a straight-line gas passage along the diffuser surface. The staggered arrangement of projections operates to further disrupt and prevent the formation of eddy currents within the diffuser as the gas passes therethrough, thereby operating to promote gas flow efficiency through the diffuser and heat exchanger.

In an example embodiment, the projections are provided in the form of convex dimples that have a rounded cross sectional profile and that project outwardly a predetermined distance from the diffuser inside wall surface. The size of the dimples and the amount that each projects from the surface can and will vary depending on the particular use application. In an example embodiment, for use with a heat exchanger having a diffuser inlet passage that is sized approximately 38 mm in diameter, that has a outwardly diverging wall section that extends from this diameter to a diameter of approximately 80 mm within a distance of approximately 30 mm, and that is engineered to handle a volumetric gas flow of approximately 0.12 cubic meters/sec, the dimples are sized having a radius of curvature of approximately 1.5 mm and extend a distance from the inside wall surface of approximately 1 mm. It is to be understood that this is but one representative example of an embodiment of the diffuser within the scope of this invention, and diffusers configured having dimples sized differently than this representative example are understood to be within the scope of this invention.

The projections can be integrally formed from the wall surface by such methods as stamping, embossing or the like. Alternatively, instead of being formed as integral elements of the diffuser wall surface, the projections can be formed from separate elements that are each attached to the diffuser inside surface. In such example embodiment, and as further discussed below, the projections operate to minimize any flow separation of gas as it is passed through the diffuser. The dimples or projections can be solid or hollow.

The projections can be shaped having a variety of different configurations, e.g., they can be round, semispherical, square, conical, triangular, rectangular, etc., or any combination thereof. In a preferred embodiment, the projections are provided in the form of rounded dimples as described above. A feature of the projections, regardless of the shape, is that they function to reduce or eliminate the occurrence of eddy flow currents along the diffuser wall surface to minimize or eliminate unwanted boundary layer separation of the gas flow passing through the diffuser.

Alternatively, rather that projections, the surface features can be provided in the form of a plurality of recessed elements, e.g., convex dimples that are recessed into the inside wall surface of the diffuser. Still further, the surface features can be provided in the form of a combination of convex projections and concave recessed elements, e.g., that are combined in an ordered or random arrangement. As noted above, regardless of their particular configuration, the surface features a preferably provided in a configuration that will function to reduce or eliminate the occurrence of eddy flow currents along the diffuser wall surface to minimize or eliminate unwanted boundary layer separation of the gas flow passing through the diffuser.

FIG. 3 schematically illustrates how the surface features disposed along the inside surface of the diffuser operates to control the flow of gas therein, and more specifically, how they operate to reduce formation of eddy currents that are known to cause unwanted boundary layer separation. More specifically, the projections operate to prevent what is otherwise a macro-separation of flow formed along the diffuser walls by a relatively large recirculation zone, and replace it with a system of micro-separations of flow formed by many relatively small recirculation zones causing reduced boundary layer separation and improved gas flow efficiency.

As best illustrated in FIGS. 4A and 4B, once the gas passes through the inlet passage 44, a portion of the gas stream 57 flows adjacent the outwardly diverging wall section 46. FIG. 4A illustrates a conventional diffuser that does not include the surface features of this invention. In this diffuser, the gas stream 57 is split into a relatively large recirculation zone 58 that is formed and exists along the wall section 46. This large recirculation zone 58 produces the above-noted macro-separation of gas within the diffuser that causes the gas stream to flow away from the diffuser wall and, thereby causes reduced gas flow distribution efficiency within the diffuser and through the heat exchanger.

FIG. 4B illustrates a diffuser of this invention comprising a plurality of the surface features provided in the form of projections 56 along the wall surface 46. As shown, the projections operate to interrupt formation of a large recirculation zone along the wall surface, and replaces it with a series of very small recirculation zones 59 that produce a system of micro-separations. The replacement of the single large macro-separation by the system of smaller micro-separations is desired as it operates to reduce the diversion of gas flow away from the wall, thereby operating to increase gas flow distribution efficiency within the diffuser and through the heat exchanger.

FIGS. 5 and 6 show another embodiment of a diffuser 60 of this invention comprising the same general elements as that disclosed above for the diffuser illustrated in FIG. 3, e.g., a flange 62 at one end 64, a neck 66 defining an inlet flow passage 68, and a outwardly diverging wall section 70 extending radially outwardly moving axially away from the inlet flow passage 68. Unlike the diffuser in FIG. 3, however, this embodiment of the diffuser has a outwardly diverging wall section that is defined by outwardly diverging sidewalls curvilinear in shape. The diffuser 60 includes a plurality of surface features 72 disposed along an inside wall surface. As noted above, the plurality of surface features 72 can be positioned along a portion of or the entire region of the inlet passage and/or outwardly diverging wall section.

It is to be understood that diffusers of this invention can be constructed comprising a plurality of surface features disposed along in inside wall surface of any shaped where it is desirable prevent boundary layer separation. Thus, the representative examples described and illustrated herein comprising conical and curvilinear inside wall sections are not intended to be a limitation on the present invention.

As noted above, the use of ribs or fins along the diffuser surface is known. However, the use of such ribs or fins are known to only affect the boundary separation of gas flow perpendicular to the gas flow direction. In contrast, the use of the surface features presented along the inside wall surface of the diffuser makes it possible to affect the gas flow in all directions within the diffuser, rather than just one. This result is especially beneficial in situations where the gas flow may be swirling or at an angle within the diffuser due to adverse conditions downstream from the diffuser. Additionally, the formation of the surface features, e.g., in the form of projections or dimples, during the process of casting the diffuser is much easier than that required for forming ribs or fins.

As noted above, it will be understood by those skilled in the art that the shape, pattern and depth of the surface features, can and will vary depending on such factors as the size and shape of the heat exchanger and diffuser, the volumetric gas flow rate, and the end use application. In one example embodiment, illustrated in FIG. 6, the diffuser 60 is shown to include projections 72 that are shaped in the form of rounded dimples that each project outwardly a distance away from the outwardly diverging wall section 70. In this particular example, the dimples 72 are arranged in three rows, moving axially away from the inlet passage 68, that each extend circumferentially around the wall section.

A first row of dimples 74 extends circumferentially around the outwardly diverging wall section 70 and is placed adjacent the inlet passage 68. The dimples in this first row are spaced apart at equal intervals from adjacent dimples in the first row. A second row of dimples 76 also extends circumferentially around the outwardly diverging wall section 70 and is placed a predetermined distance axially away from the first row of dimples 74. The dimples in this second row are spaced apart at equal intervals from adjacent dimples in the second row. Additionally, the dimples in the second row are positioned in a manner that is staggered from the dimples in the first row, such that each dimple in the second row is positioned axially between two dimples in the first row.

A third row of dimples 78 also extends circumferentially around the outwardly diverging wall section 70 and is placed a predetermined distance axially away from the second row of dimples 76. The dimples in this third row are spaced apart at equal intervals from adjacent dimples in the third row. Additionally, the dimples in the third row are positioned in a manner that is staggered from the dimples in the second row, such that each dimple in the third row is positioned axially between two dimples in the second row and is in axial alignment with a dimple in the first row. In such example embodiment, the first, second and third rows of dimples are equally spaced axially from one another.

For the purpose of helping to optimize gas flow within the diffuser 60, the first row of dimples is positioned along on the inside surface of the outwardly diverging wall section 70 at a point where the wall section 70 starts to diverge sharply, e.g., when the tangent to the wall is at an angle greater than about 20 degrees from the gas flow direction. The third row of dimples 78 is positioned along the inside surface of the outwardly diverging wall section 70 at a point where the wall section 70 begins to converge again as it extends towards the end of the diffuser that connects with the shell. The second row of dimples 76 is interposed along the surface of the outwardly diverging wall section 70 between the first row of dimples 74 and the third row of dimples 78.

While diffusers have been described and illustrated as including surface features positioned circumferentially around an entire portion of a diffuser inside wall surface, it is to be understood that diffusers of this invention can be constructed having surface features that are positioned at only selected regions of an inside wall surface, e.g., not provided in rows that extend completely around the inside wall surface. For example, for those applications known to have an asymmetric gas flow patters entering the diffuser, diffusers of this invention may comprise the surface features positioned along only that part or region of the diffuser inside surface that is likely to encounter the gas flow.

A feature of heat exchangers comprising a diffuser having the surface features described above is that such surface features operate to reduce or eliminate the formation of a large recirculation zone along the inside wall surface, thereby reducing or eliminating the occurrence of macro boundary layer separation within the gas flow stream passing therethough. The use of such surface features, thus enables the heat exchanger designer to construct a compact heat exchanger in a manner that avoids unwanted pressure losses, that improves gas flow distribution, and that improved and thermal heat transfer inefficiencies within the heat exchanger. Further, the use of such surface features, e.g., when provided in the form of dimples, enables the control of gas flow to reduce or prevent large recirculation zones in multiple directions, thereby operating to further optimize gas flow through the heat exchanger. A still other feature of diffusers of this invention, when such surface features are provided in the form of projections such as dimples, is that they can be formed in a cost effective manner even when the diffuser surface has a complex shape, e.g., by molding process.

Although the concept of using a plurality of surface features, to reduce or prevent boundary layer separation of gas flow, has been disclosed and illustrated within the context of a heat exchanger, e.g., within a diffuser positioned at the inlet of such heat exchanger, it is to be understood that the invention concept of using such surface features can additionally be applied to avoid unwanted boundary layer separation and optimize gas or fluid flow in other applications that may not involve or exist within a heat exchanger.

FIG. 7 illustrates a tube bend 80 that is used for the transport of gas or fluid therethrough. In such tube bend, the issue of boundary layer separation may exist due to the creation of eddy currents and related large recirculation zones that exist along a region of the inside wall surface 82 adjacent an inside radius 84 of the tube. In such application, the placement of the surface features described above along such region 82 can operate to reduce or eliminate eddy current formation in this region, thus operating to reduce or eliminate boundary layer separation at this region and optimize gas or fluid passage therethrough. The same is true for diffusers that incorporate a bend or other type of geometric feature, in addition to that described above, that would cause boundary layer separation.

Although the invention as described and illustrated above has been presented in the context of a shell and tube-type heat exchanger, it is to be understood that the dimpled diffuser of this invention can be used with other types of heat exchangers and any gas or fluid conducting volume whose shape might create eddy losses, and where it is desirable to improve flow distribution and reduce those eddy losses. For example, the dimpled diffusers of the present invention may be used with intake manifolds, charge air ducting and the like. Such embodiments are intended to be within the scope of this invention. Additionally, while a particular embodiment of the diffuser of this invention has been described and illustrated, it is to be understood that modifications and variations of this configuration may be apparent to those skilled in the art, and that such modifications and variations are intended to be within the scope of this invention. 

1. A diffuser that is connected to a heat exchanger for receiving a gas or fluid and directing the same to the heat exchanger, the diffuser having a body that comprises: a gas or fluid inlet passage at one end of the body and extending a distance therein; an outwardly diverging wall section extending radially away from the inlet passage with axial distance from the inlet passage; and an outlet extending axially away from the outwardly diverging wall section; wherein the inlet passage, the outwardly diverging wall, and outlet each have an inside surface that defines an interior through which a gas or fluid passes in a flow direction from the inlet passage to the outlet, and wherein at least a section of the inside surface includes a plurality of individual surface features that are disposed therealong.
 2. The diffuser as recited in claim 1 wherein the individual surface features are projections that each project outwardly a distance therefrom and that have a radiused cross sectional profile.
 3. The diffuser as recited in claim 1 wherein the plurality of individual surface features are positioned along at least a region of the inside surface of the outwardly diverging wall section.
 4. The diffuser as recited in claim 3 wherein the plurality of individual surface features occupy at least about 10 percent of the surface area of the outwardly diverging wall section.
 5. The diffuser as recited in claim 1 wherein the plurality of individual surface features are arranged within a plurality of rows, and wherein each row comprises a plurality of the individual surface features.
 6. The diffuser as recited in claim 5 wherein the surface features in at least one row are arranged so they are not axially aligned with the surface features in an adjacent row.
 7. The diffuser as recited in claim 1 wherein the outwardly diverging wall section is conical, wherein the plurality of individual surface features are in the form of dimples positioned along an inside surface of the outwardly diverging wall section and projecting outwardly a distance therefrom, and wherein the dimples are provided in two or more rows that each extend around the outwardly diverging wall section.
 8. The diffuser as recited in claim 1 wherein the outwardly diverging wall section is curvilinear, wherein the plurality of individual surface features are in the form of dimples positioned along an inside wall surface of the outwardly diverging wall section and projecting outwardly a distance therefrom, and wherein the dimples are provided in two or more rows that each extend around the outwardly diverging wall section.
 9. A heat exchanger comprising a plurality of gas or fluid passages disposed within a shell, the plurality of gas or fluid passages being defining a first gas or fluid flow passage through the heat exchanger, and the shell defining a second gas or fluid flow passage through the heat exchanger that is separate from the first gas or fluid flow passage, and further comprising a diffuser that is connected to the heat exchanger and in gas or fluid flow communication with the first gas or fluid flow passage, the diffuser comprising: an inlet passage for receiving a gas or fluid stream and directing the same to the first gas or fluid flow passage; an outwardly diverging wall section extending away from the inlet passage; and an outlet that extends away from the outwardly diverging wall section and that is connected with the heat exchanger; wherein the outwardly diverging wall section includes a plurality of projections disposed along an inside surface and that extend outwardly a distance therefrom, wherein the projections are provided in two or more rows, and wherein the projections in each row are separated from one another.
 10. The heat exchanger as recited in claim 9 wherein the projections have a radiused cross sectional profile.
 11. The heat exchanger as recited in claim 9 wherein the plurality of projections occupy at least about 10 percent of the surface area of the outwardly diverging wall section.
 12. The heat exchanger as recited in claim 9 wherein the projections in at least one row are arranged so they are not axially aligned with the projections in an adjacent row.
 13. The heat exchanger as recited in claim 9 wherein the outwardly diverging wall section is conical, and wherein the plurality of projections are dimples having a radiused sued cross sectional profile.
 14. The heat exchanger as recited in claim 9 wherein the outwardly diverging wall section is curvilinear, and wherein the plurality of projections are dimples having a radiused cross sectional profile.
 15. A method of reducing boundary layer separation in a gas or fluid flowing through a heat exchanger comprising an inlet diffuser, the diffuser having an inlet passage, an outwardly diverging side wall extending from the inlet passage, and an outlet extending from the outwardly diverging side wall, the method comprising the steps of: forming a plurality of individual surface features on at least a portion of an inside surface of the outwardly diverging side wall; and flowing a gas or fluid through the inlet passage, over the surface features and through the outlet, whereby plurality of individual surface features interrupts the creation of eddy currents along the outwardly diverging side wall that reduces boundary layer separation the gas or fluid.
 16. The method as recited in claim 15 wherein during the step of forming, the surface features are provided in the form of projections that each extend a distance from the inside surface, wherein the projections are provided in two or more rows, and wherein the projections within each row are separate from one another.
 17. The method as recited in claim 15 wherein during the step of forming, the projections are provided in the form of dimples that each extend a distance from the inside surface and that have a radiused cross sectional profile.
 19. A method of making an inlet gas or fluid diffuser for use with a heat exchanger, the method comprising the steps of: forming a diffuser body having a gas or fluid inlet passage at one end of the body and extending a distance therein, an outwardly diverging wall section extending radially away from the inlet passage with axial distance from the inlet passage, and an outlet extending axially away from the outwardly diverging wall section; modifying a inside surface of at least a portion of the outwardly diverging wall section to include a plurality of individual surface features disposed therealong for reducing boundary layer separation of gas or fluid that is disposed thereover. 